JPH075971B2 - Method for producing alloy electroplated steel sheet for deep drawing with excellent impact peel resistance after painting - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing an alloy electroplated steel sheet for deep drawing which is excellent in impact peeling resistance after painting, and more specifically, as a steel sheet for automobiles, Zn-Ni-based alloy plating or Zn-Fe-based alloy plating is applied due to rust. No distorted pattern occurs on the molded product after painting, and the plating has excellent impact peeling resistance. The present invention relates to a method for manufacturing an alloy electroplated steel sheet for deep drawing.

[Prior Art] Generally, Zn-Ni alloy plating, Zn-Fe alloy plating, etc.
Zn-based alloy electroplated steel sheets are superior in corrosion resistance, corrosion resistance after coating, spot weldability, etc. to conventional electrogalvanized steel sheets and hot-dip galvanized steel sheets. Is widely used for.

However, this steel sheet has a problem with the prior art as described below in press forming or use.

(1) After the electroplating treatment, an organic coating treatment is performed to further increase the corrosion resistance, but since the organic coating treatment temperature reaches 150 ° C, it is distorted by carbon atoms existing freely in the steel. Aging occurs and not only deteriorates the material of the steel sheet before press molding, but also distorts the appearance of the product because a distortion pattern is generated on the surface of the molded product.

(2) The corrosion resistance of the Zn-based alloy electroplated steel sheet is good when the main component of the plating layer is an intermetallic compound, but this plating layer is harder than zinc plating that has been widely used in the past. Since it is brittle, the deep drawability (total elongation, n value, γ value) after plating is significantly deteriorated as compared with the base material before plating, making press molding difficult.

(3) In relation to (2) above, when a Zn-based alloy electroplated steel sheet is coated and then an impact force is applied, the plating layer itself peels from the interface of the base steel sheet, and this peeled surface Since the plating layer does not remain, scab corrosion or perforation corrosion starting from this point easily occurs, and this peeling phenomenon occurs only when coating is performed, and the coating film is thick. The higher the hardness, the lower the ambient temperature.

As means for solving such problems of the conventional technology (1)
Regarding (2), it is known to make a steel sheet containing extremely strong carbonitride-forming elements such as Ti and Nb in extremely low C due to degassing of steel. However, when this steel sheet contains Ti, the chemical conversion treatment property of the steel sheet surface is deteriorated and the cost is increased. Regarding (3), for example, a method of coating a steel sheet with an element nobler than Zn in a metallic state before plating (Japanese Patent Laid-Open No. 60-128286), a metallic state with a Vickers hardness of 300 or more as a lower layer of plating The method of coating Fe of the above (Japanese Patent Laid-Open No. 61-019794) has been proposed.

However, in order to manufacture these steel sheets, equipment for plating these underlayers is required in addition to the Zn-Ni alloy electroplating bath, and it cannot be carried out immediately. Also, C
u, Sn, Cd, underlying plating of Pb adversely affects corrosion resistance of Zn-Ni alloy electroplated layer, further, Fe plating also not contribute to corrosion resistance, plating 1.5 to 20 / m ² are corrosion It is useless in terms of sex.

[Problems to be Solved by the Invention] As described above, the present invention addresses impact peel resistance after coating in the prior art by changing the composition of the components and the content ratio in the plating layer. However, in view of the problem that the impact peeling resistance and deep drawability after coating are not sufficiently satisfied, the present inventor conducted diligent research,
As a result of repeated examinations, a solution was found on the manufacturing surface of the bare plate before plating, the impact peeling resistance of the coating film after coating was improved, and the strain aging and deep drawability were also excellent. We have developed a method for manufacturing alloy electroplated steel sheets for deep drawing that have excellent impact resistance.

[Means for Solving Problems] A feature of the method for producing an alloy electroplated steel sheet for deep drawing having excellent impact peel resistance after coating according to the present invention is that C 0.02 to 0.06 wt% and Mn 0.10 ~ 0.30wt%, P ≤ 0.005wt
%, S ≤ 0.005 wt%, Al 0.03 to 0.07 wt%, N 0.003 to 0.008 wt
% Of steel, the balance of which is Fe and unavoidable impurities, is soaked and maintained at a temperature of 1200 ° C or higher to a finishing temperature of Ar _{3 to} 900.
After continuous hot rolling at ℃, cool at a cooling rate of 50 to 120 ℃ / sec until winding, wind at a winding temperature of 600 ℃ or less, cold roll by the usual method, and recrystallization temperature or higher. In the above, box annealing is performed, and then alloy electroplating treatment and organic coating treatment are performed.

The method for producing an alloy electroplated steel sheet for deep drawing which is excellent in impact resistance after coating according to the present invention will be described in detail below.

First, a description will be given of the components and the content ratio of the steel used in the method for producing an alloy electroplated steel sheet for deep drawing having excellent impact peel resistance after coating according to the present invention.

C is an element that affects deep drawability, and the smaller the amount, the more preferable. However, the free C content (solid solution C content) existing in the base plate in order to prevent the distortion pattern generated on the surface of the steel sheet during forming after coating. ) Should be as low as possible, and the content should be 0.
If it is less than 02 wt%, precipitation of cementite is small, so solid solution C
If the content is large, and if the content exceeds 0.06 wt%, not only the deep drawability deteriorates but also the increase in strength and the ductility are impaired. Therefore, the C content is 0.02 to 0.06 wt%.

Mn is also hardened when it becomes large like C, leading to deterioration of deep drawability and ductility. The lower the Mn is, the more preferable it is, but if it is too low, hot brittleness due to S occurs, and the content is 0.10 wt. If it is less than%, the above effect cannot be expected, and it is 0.
If it is contained in excess of 30 wt%, the above-mentioned unfavorable effects are exerted. Therefore, the Mn content is 0.10 to 0.30 wt%.

P is an important element, and as the P content increases, as described below, it becomes an oxide and concentrates on the surface of the steel sheet, which impairs impact peel resistance, and even if a small amount of P hardens. Deep drawing ability because of its high performance and segregation at the grain boundaries hinders grain growth of the crystal,
Since the ductility is impaired, the P content should be 0.005 wt% or less.

S is an important element like P, and as described below, the impact peeling resistance is deteriorated when the S content is increased.
The content is 0.005 wt% or less.

Next, the contents of P and S will be described in more detail below.

Steels used as components other than P and S in a method for producing an alloy electroplated steel sheet for deep drawing having excellent impact peeling resistance after coating according to the present invention (sometimes referred to as the production method of the present invention). A vacuum-melted steel outside the above range was made into a slab with a thickness of 30 mm in a laboratory, and then a hot rolled steel sheet with a thickness of 3.2 mm was manufactured under the hot rolling conditions within the range of the manufacturing method of the present invention.

This hot-rolled steel sheet was pickled and cold-rolled to a thickness of 0.8 mm, box-annealed at 690 ° C for 3 hours, Zn-Ni alloy electroplating, organic coating treatment, and automobile coating (total coating thickness) FIG. 1 shows the relationship between the deep drawability (γ value), the impact peel resistance of the coating film after automobile coating, and the P and S contents in steel.

The coating conditions and impact peelability test were conducted by the following methods.

The produced test piece was subjected to the dipping method phosphate treatment → cationic electrodeposition coating → intermediate coating → top coating according to the automobile coating process.

The coating film thickness was 100 μm in total of the three coats. After cooling these coated test pieces to -20 ° C, the coated surface was scratched with a gravelometer.

The conditions of the gravelometer are as follows.

Air pressure 4kgf / cm ² Stone type Granite Stone diameter 7.4〜11.1mm Stone amount 100g After this test, the coating film released from the steel sheet was removed with adhesive tape, and the coating peeled from the interface between the plating layer and the base steel sheet. The maximum diameter of the part was measured.

The measurement was performed at 10 points from the largest diameter, and the average value was shown.

As is clear from FIG. 1, when the S content is 50 ppm or less, when the P content is 50 ppm or less, particularly when it is 40 ppm or less, the plating peeling diameter becomes small and the impact peeling resistance is improved, but When the P content and the S content were 50 ppm or more, the plating peeling diameter was large and the impact peeling resistance was not improved. The deep drawability (γ value) improves as the P content decreases regardless of the S content. However, when the P content is 50 ppm or less, the effect of the S content appears, and when the S content is 50 ppm or less, The drawability (γ value) is significantly improved. Further, when the alloy plating treatment is carried out, the deep drawability (γ value) is deteriorated as compared with the original plate which is not subjected to the alloy plating treatment. A high deep drawability (γ value) of 1.8 or more can be secured.

As described above, it was found that by positively reducing the P content and the S content contained in the steel sheet, it is possible to manufacture an alloy electroplated steel sheet having excellent impact peel resistance and deep drawability.

Then, by making the P content and the S content very small,
The detailed reason why these characteristics are improved is not clear at present, but if the P content is large, the oxides are concentrated on the surface of the steel sheet and concentrate, and if the S content is large, the non-metallic inclusions in the steel sheet are generated. It is considered that the impact peeling resistance is deteriorated in any case by increasing the amount, and the deep drawability is greatly hardened by the hot rolling conditions together with the P content and the S content.

Al is usually contained as a deoxidizing agent, but in the production method of the present invention, it is an element to be contained in order to improve the deep drawability, and if the content is less than 0.03 wt%, this effect cannot be expected, and If it is contained in an amount of more than 0.07 wt%, the hardening will be saturated, and further the crystal grains will become finer and harder and the deep drawability will be deteriorated. Therefore, the Al content is 0.03 to 0.07 wt%.

It is known that N is generally preferable as the content is smaller because of the problem of strain aging. However, in order to obtain characteristics as an aluminum killed steel sheet for deep drawing, if the content is less than 0.003, such an effect is expected. Not possible, 0.008wt%
If it is contained in a large amount over the range, the amount of solute N increases, which leads to deterioration of ductility due to strain aging. Therefore, the N content is 0.003 to 0.008 wt%.

The method for melting steel used in the manufacturing method of the present invention described above may be any method and is not limited, and may be any of a converter, open hearth furnace, electric furnace and the like.

In the production method of the present invention, the above steel is made into a slab by an ingot-segmentation method and a continuous casting method, the slab is hot-rolled under predetermined conditions, and after cold-rolling,
Box annealing is performed.

Furthermore, in the manufacturing method of the present invention, the hot rolling conditions, the cold rolling conditions and the annealing conditions for further enhancing the deep drawability of the alloy electroplated steel sheet will be described below.

The heating temperature of the slab prior to continuous hot rolling is set to 1200 ° C. or higher so that Al and N are dissolved in steel as much as possible.
If the finishing temperature is less than ₃ Ar points, the (200) crystal plane will increase.
At temperatures above 900 ° C, the crystal grains of the hot-rolled steel sheet become large, and all of these deteriorate the deep drawability of the annealed sheet, so the temperature range is Ar _{3 to} 900 ° C.

After that, the coil is cooled until winding, but the cooling rate during that time needs to be 50 to 120 / sec faster than usual. This is to prevent the growth of trace crystal grains obtained immediately after finish rolling and to leave a large amount of solute Al and solute N content.
If it is less than / sec, the above phenomenon cannot be obtained, the deep drawability is deteriorated, and special equipment is required to obtain a cooling rate of more than 120 ° C / sec.

Next, if the coil winding temperature is too high, solid solution Al and solid solution N
Since it precipitates as N and deteriorates the deep drawability of the annealed plate, it is preferably lower, and is set to 600 ° C or lower.

In order to improve the impact peeling resistance of the coating film, the content and proportion of the steel in the production method of the present invention are set to P in the steel.
Although a small amount of S content and S content are contained, this also contributes to the improvement of deep drawability and ductility. However, with this alone, it is difficult to secure the deep drawability of the plated steel sheet, and if the P content and the S content are small, the crystal grains of the hot-rolled sheet are likely to become large, so the heat of the manufacturing method of the present invention The inter-rolling conditions are aimed at making the grains finer, which further improves the deep drawability. The coil thus obtained is pickled and then cold rolled. The cold rolling rate is preferably 70 to 80% in order to obtain deep drawability.

Then, the cold rolled plate is box annealed by a tight coil or an open coil. The heating rate is not particularly limited, but deep drawability can be obtained if it is a usual 30 to 60 ° C./hr.
The annealing temperature is set to the recrystallization temperature to 700 ° C, but if the temperature is lower than the recrystallization temperature, deep drawability cannot be obtained, and if the temperature exceeds 700 ° C, solid solution C remains in the annealed steel sheet, so coating Not only does it cause strain aging due to the treatment and deteriorates ductility,
Impair the appearance of the product surface.

After that, Zn-Ni-based, Zn-Fe-based, Zn-Co-based, etc. Zn-based alloy electroplating treatment is subsequently performed on the surface of the steel sheet, followed by chromate treatment and coating treatment, but these conditions are particularly restricted. is not.

[Examples] A method for producing an alloy electroplated steel sheet for deep drawing having excellent impact peel resistance after coating according to the present invention will be described.

Example A small molten steel of the steel used in the manufacturing method of the present invention and the method of the comparative example having the components and content ratios shown in Table 1 was used, and the thickness was set to 30 mm.

This was held at a heating temperature of 1050 ° C. and 1250 ° C. for 30 minutes, then finished at a finishing temperature of 800 to 890 ° C., then shower cooled at 10 to 95 ° C./sec, and wound at temperatures of 500 ° C. and 680 ° C.

This hot-rolled steel sheet was pickled, cold-rolled at a rolling reduction of 75% to a thickness of 0.8 mm, heated at a heating rate of 40 ° C / hour, and then annealed at 690 ° C and 750 ° C for 3 hours for box annealing. , Then 0.
8% temper rolling was performed.

The temper-rolled steel sheet thus produced was subjected to galvanic alloying, organic coating treatment, and was examined for tensile properties and deep drawability. Furthermore, for steel sheets coated for automobiles, impact peel resistance was evaluated. A survey was conducted.

The results are shown in Table 2.

Steels A, Al, and BD are steels used in the manufacturing method of the present invention, and steels E to J are steels used in Comparative Examples.

Steel E has a C content, steel F has a Mn content, steel G has an S content, and steel I and steel J have an Al content and an N content in the ranges of the content ratios of the steels used in the production method of the present invention, respectively. Outside.

From Table 2, the organic film produced by the manufacturing method of the present invention is alloy electroplated steel sheet, which has a low YS (yield stress) of 18 kgf / mm ² or less, 4
With a high El (total elongation) of 7% or more and a high γ value of 1.7 or more, YPEl (yield point elongation) does not occur due to the organic film treatment, and further,
Since the plating peeling diameter after the impact test is as small as 3 mm or less, the perforated corrosion is also small, which indicates that both the deep drawability and the impact peeling resistance are excellent.

On the other hand, outside the range of the steel of the manufacturing method of the present invention, the γ value for steel E whose manufacturing conditions are within the range of the manufacturing method of the present invention,
Although the releasability is good, strain aging also occurs due to the film treatment, and YPEl is generated and YS and El are deteriorated. Steel F, I,
J has a good peeling diameter, but is inferior in deep drawability, and steels G and H
Has a large peeling diameter and is inferior in corrosion resistance. Each of the comparative steel sheets is inferior in deep drawability and fire shock peeling resistance, and cannot have both properties.

Next, using Steel A, the effects of hot rolling and annealing conditions were examined. Steel A2 is slab heating temperature, A3 is finishing temperature, steel
A4 is the cooling rate, steel A5 is the coiling temperature, steel A6 is the annealing temperature is outside the scope of the manufacturing method of the present invention, respectively, these comparative steels are excellent in impact peel resistance steel A2-A5 Low γ value, poor deep drawability, steel A6 is excellent in deep drawability and impact debonding resistance, but has a high content of solute C due to high annealing, strain aging occurs due to organic film treatment, YS, It can be seen that El and n values are deteriorated and YPEl is generated, and none of them satisfy the necessary conditions.

[Effects of the Invention] As described above, the method for producing an alloy electroplated steel sheet for deep drawing having excellent impact peeling resistance after coating according to the present invention has the above-described configuration, and thus is contained in the steel used. It is possible to improve impact peel resistance after coating by adjusting the contained components and content ratios, and further, it is possible to produce a steel sheet having excellent deep drawability and strain aging resistance. It is a thing.

[Brief description of drawings]

Fig. 1 shows the deep drawability (γ value), impact peeling resistance (plating peeling resistance), and P content in the steel sheet, and the S content of the sheet that has been subjected to automobile coating process after alloy electroplating and organic film treatment. There is a graph showing the relationship with quantity.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-59-13030 (JP, A) JP-A-60-221526 (JP, A) JP-A 61-19794 (JP, A) JP-A-60- 128286 (JP, A)

Claims (1)

[Claims] 1. C 0.02 to 0.06 wt%, Mn 0.10 to 0.30 wt%, P
≤0.005wt%, S ≤0.005wt%, Al 0.03-0.07wt%, N 0.003-0.008wt
% Of steel, the balance of which is Fe and unavoidable impurities, is soaked and maintained at a temperature of 1200 ° C or higher to a finishing temperature of Ar _{3 to} 900.
After continuous hot rolling at ℃, cool at a cooling rate of 50 to 120 ℃ / sec until winding, wind at a winding temperature of 600 ℃ or less, cold roll by the usual method, and recrystallization temperature or higher. A method for producing an alloy electroplated steel sheet for deep drawing which is excellent in impact peeling resistance after painting, characterized by performing box annealing in, and then performing alloy electroplating treatment and organic coating treatment.